| Black medic (Medicago lupulina, cv George), a self-regenerating legume cover crop, was evaluated to quantify its soil nitrogen supply potential using two no-till long-term field experiments in Manitoba (established in 2000) and Saskatchewan (established in 2001) with the rotation of flax ( Linum usitatissimum)-wheat (Triticum aestivum)-oat (Avena saliva). The main plot is presence of medic while the subplot is N fertilizer rate; two levels at Winnipeg (with and without fertilizer) and three levels at Indian Head (20%, 60% and 100% N fertilizer). No previous study has been done in a prairie no-till, continuous cropping system to assess the N benefit of black medic.; The first of two experiments was a field study conducted in 2006, to determine the effect of medic on soil NO3--N and on crop N content, biomass accumulation, N uptake and grain yield. The second study tested the N supply potential of surface soil (015 cm) from the field experiments in 2005 and 2006 using a bioassay. According to field experiment results, medic little influence was on crop performance and significant on soil NO3--N at tillering and maturity stages. Grain yield reduction due to medic was observed at one of three instances. At Indian Head, medic increased total N (plant + soil) available at maturity by 48 kg ha-1. Lack of significant medic effect at Winnipeg was attributed to drought conditions. In the bioassay studies, medic soil from both sites showed a highly significant increase in N supply potential: Winnipeg 33 kg N ha-1 and 38 kg N ha-1 in 2005 and 2006, respectively and Indian Head 32 kg N ha-1 and 48 kg N ha-1 in 2005 and 2006, respectively. Unlike in the field experiment, medic soils from both sites showed equal measure of N supply potential in the bioassay due to no differences in growing condition. Lack of significant interaction between medic and N fertilizer regimes demonstrated the adaptability of the black medic cover crop in cropping systems with different N fertilizer management. The major conclusion of this study is that the black medic significantly increases N-supply potential of soils by 38 kg N ha-1 under favorable prairie growing conditions; therefore, farmers could save a portion of N fertilizer cost. |
